The retrieval of desired fluids, such as hydrocarbon based fluids, is pursued in subsea environments. Production and transfer of fluids from subsea wells relies on subsea installations, subsea flow lines and other equipment. Additionally, preparation and servicing of the subsea well relies on the ability to conduct subsea intervention work. A big challenge in subsea intervention work is controlling pressure so that pressurized borehole fluids in the subsea well are contained within the borehole during intervention operations. Subsea intervention work involves numerous challenges not normally faced when working on land wells or offshore platforms. In most cases, intervention in subsea wells is performed from a floating platform or ship by extending the borehole to a surface location by a tensioned riser. This approach allows pressurized borehole fluids to move upwardly to the surface through the riser which can span hundreds or thousands of feet of sea water. The cost of such platforms is high, however, and the availability of vessels capable of adequately performing this type of intervention work is limited.
In shallow waters, subsea intervention work can be performed with a specially equipped vessel having subsea lubricators, subsea pressure control equipment, and wave motion compensating systems. In most cases, guide wires extending from a wellhead all the way to the vessel combined with the aid of professional divers is required. Additionally, this approach requires that equipment is conveyed and guided from the vessel to the subsea installation through open waters. Once the subsea lubricator is connected to the subsea installation and the tools are inside, the conveyance cable remains exposed to open waters. Additionally, pressure control must be exercised at the seabed. Because existing non-rig intervention capability is limited to shallow water wireline and slickline operations, most intervention on subsea wells is currently performed with expensive and scarce heavy drilling units.
When exercising pressure control, borehole fluids are kept separated by seals formed during certain well servicing operations. In U.S. Pat. No. 4,905,763, for example, separation of borehole fluids is maintained during logging operations by a sealing nipple and a stuffing box assembly. The assembly is lowered down through a riser extending between a platform and a blowout preventer stack. In another application described in U.S. Pat. No. 4,951,745, a hydraulically actuated stuffing box is mounted on top of an underwater lubricator assembly to seal against a line during well service operations.